Throughout his teen years, Muhammad worked as a shepard to pay his keep to his uncle. Muhammad was orphaned as a boy and lived with his uncle in Mecca. As a shepard, Muhammad travelled far. He occassionally traveled and worked with a merchant named Abu Talib, and the pair traveled as far north as Syria. While on his travels as a shepard and with his merchant friend, Muhammad was exposed to a variety of other beliefs, including monotheism (in the forms of Christianity and Judaism, likely encountered from European merchants in Turkey and Syria on the Silk Road). Islam shares many similarities to these other monotheistic religions, and it is likely that Muhammad's interaction with Christians and Jews had an impact on Islam and its spread only decades later.
In his twenties, Muhammad enters the service of a merchant named Khadija, Muhammad's distant cousin and a wealthy Meccan widow. In 595, the pair marry. Later, when Muhammad receives his visions, it is Khadija that encourages his to listen to Allah and follow the visions. Khadija had a major impact on the birth of Islam, so his business relationship-turned-marriage to Khadija was a major event in early Islamic history.
Muhammad's first revelation in 610 was a key event as well. Mecca's polytheism and idol worship disturbed the worldly Muhammad. During a series of fasts and meditations, Muhammad was visited by Allah. While he at first did not tell anyone about these visions, it was these first interactions with Allah that encouraged Muhammad to take his message about Islam public in Mecca.